[RFC] Integrate RPMSG/SMD into WWAN subsystem

[RFC] Integrate RPMSG/SMD into WWAN subsystem

[Stephan Gerhold]

Hi Loic, Hi Bjorn,

I've been thinking about creating some sort of "RPMSG" driver for the
new WWAN subsystem; this would be used as a QMI/AT channel to the
integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.

It's easy to confuse all the different approaches that Qualcomm has to
talk to their modems, so I will first try to briefly give an overview
about those that I'm familiar with:

---
There is USB and MHI that are mainly used to talk to "external" modems.

For the integrated modems in many Qualcomm SoCs there is typically
a separate control and data path. They are not really related to each
other (e.g. currently no common parent device in sysfs).

For the data path (network interface) there is "IPA" (drivers/net/ipa)
on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
I have a driver for BAM-DMUX that I hope to finish up and submit soon.

The connection is set up via QMI. The messages are either sent via
a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).

This gives a lot of possible combinations like BAM-DMUX+RPMSG
(MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).

Simply put, supporting all these in userspace like ModemManager
is a mess (Aleksander can probably confirm).
It would be nice if this could be simplified through the WWAN subsystem.

It's not clear to me if or how well QRTR sockets can be mapped to a char
device for the WWAN subsystem, so for now I'm trying to focus on the
standalone RPMSG approach (for MSM8916, MSM8974, ...).
---

Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
(drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
I just took that over from someone else. Realistically speaking, the
current approach isn't too different from the UCI "backdoor interface"
approach that was rejected for MHI...

I kind of expected that I can just trivially copy some code from
rpmsg_char.c into a WWAN driver since they both end up as a simple char
device. But it looks like the abstractions in wwan_core are kind of
getting in the way here... As far as I can tell, they don't really fit
together with the RPMSG interface.

For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
(non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
get notified when the TX queue is full or no longer full so I can call
wwan_port_txon/off().

Any suggestions or other thoughts?

Thanks in advance!
Stephan

[1]: https://elixir.bootlin.com/linux/latest/source/drivers/rpmsg/rpmsg_core.c#L151

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Loic Poulain]

Hi Stephan,

On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> Hi Loic, Hi Bjorn,
>
> I've been thinking about creating some sort of "RPMSG" driver for the
> new WWAN subsystem; this would be used as a QMI/AT channel to the
> integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
>
> It's easy to confuse all the different approaches that Qualcomm has to
> talk to their modems, so I will first try to briefly give an overview
> about those that I'm familiar with:
>
> ---
> There is USB and MHI that are mainly used to talk to "external" modems.
>
> For the integrated modems in many Qualcomm SoCs there is typically
> a separate control and data path. They are not really related to each
> other (e.g. currently no common parent device in sysfs).
>
> For the data path (network interface) there is "IPA" (drivers/net/ipa)
> on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> I have a driver for BAM-DMUX that I hope to finish up and submit soon.
>
> The connection is set up via QMI. The messages are either sent via
> a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
>
> This gives a lot of possible combinations like BAM-DMUX+RPMSG
> (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
>
> Simply put, supporting all these in userspace like ModemManager
> is a mess (Aleksander can probably confirm).
> It would be nice if this could be simplified through the WWAN subsystem.
>
> It's not clear to me if or how well QRTR sockets can be mapped to a char
> device for the WWAN subsystem, so for now I'm trying to focus on the
> standalone RPMSG approach (for MSM8916, MSM8974, ...).
> ---
>
> Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> I just took that over from someone else. Realistically speaking, the
> current approach isn't too different from the UCI "backdoor interface"
> approach that was rejected for MHI...
>
> I kind of expected that I can just trivially copy some code from
> rpmsg_char.c into a WWAN driver since they both end up as a simple char
> device. But it looks like the abstractions in wwan_core are kind of
> getting in the way here... As far as I can tell, they don't really fit
> together with the RPMSG interface.
>
> For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> get notified when the TX queue is full or no longer full so I can call
> wwan_port_txon/off().
>
> Any suggestions or other thoughts?

It would be indeed nice to get this in the WWAN framework.
I don't know much about rpmsg but I think it is straightforward for
the RX path, the ept_cb can simply forward the buffers to
wwan_port_rx. For tx, simply call rpmsg_trysend() in the wwan tx
callback and don't use the txon/off helpers. In short, keep it simple
and check if you observe any issues.

And for sure you can propose changes in the WWAN framework if you
think something is missing to support your specific case.

Regards,
Loic

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Stephan Gerhold]

Hi Loic,

On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > I've been thinking about creating some sort of "RPMSG" driver for the
> > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> >
> > It's easy to confuse all the different approaches that Qualcomm has to
> > talk to their modems, so I will first try to briefly give an overview
> > about those that I'm familiar with:
> >
> > ---
> > There is USB and MHI that are mainly used to talk to "external" modems.
> >
> > For the integrated modems in many Qualcomm SoCs there is typically
> > a separate control and data path. They are not really related to each
> > other (e.g. currently no common parent device in sysfs).
> >
> > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> >
> > The connection is set up via QMI. The messages are either sent via
> > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> >
> > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> >
> > Simply put, supporting all these in userspace like ModemManager
> > is a mess (Aleksander can probably confirm).
> > It would be nice if this could be simplified through the WWAN subsystem.
> >
> > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > device for the WWAN subsystem, so for now I'm trying to focus on the
> > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > ---
> >
> > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > I just took that over from someone else. Realistically speaking, the
> > current approach isn't too different from the UCI "backdoor interface"
> > approach that was rejected for MHI...
> >
> > I kind of expected that I can just trivially copy some code from
> > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > device. But it looks like the abstractions in wwan_core are kind of
> > getting in the way here... As far as I can tell, they don't really fit
> > together with the RPMSG interface.
> >
> > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > get notified when the TX queue is full or no longer full so I can call
> > wwan_port_txon/off().
> >
> > Any suggestions or other thoughts?
> 
> It would be indeed nice to get this in the WWAN framework.
> I don't know much about rpmsg but I think it is straightforward for
> the RX path, the ept_cb can simply forward the buffers to
> wwan_port_rx.

Right, that part should be straightforward.

> For tx, simply call rpmsg_trysend() in the wwan tx
> callback and don't use the txon/off helpers. In short, keep it simple
> and check if you observe any issues.
> 

I'm not sure that's a good idea. This sounds like exactly the kind of
thing that might explode later just because I don't manage to get the
TX queue full in my tests. In that case, writing to the WWAN char dev
would not block, even if O_NONBLOCK is not set.

But I think you're right that it's probably easiest if I start with
that, see if I can get anything working at all ...

> And for sure you can propose changes in the WWAN framework if you
> think something is missing to support your specific case.
> 

... and then we can discuss that further on a RFC PATCH or something
like that. Does that sound good to you?

Thanks!
Stephan

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Loic Poulain]

Hi Stephan,

On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> Hi Loic,
>
> On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > I've been thinking about creating some sort of "RPMSG" driver for the
> > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > >
> > > It's easy to confuse all the different approaches that Qualcomm has to
> > > talk to their modems, so I will first try to briefly give an overview
> > > about those that I'm familiar with:
> > >
> > > ---
> > > There is USB and MHI that are mainly used to talk to "external" modems.
> > >
> > > For the integrated modems in many Qualcomm SoCs there is typically
> > > a separate control and data path. They are not really related to each
> > > other (e.g. currently no common parent device in sysfs).
> > >
> > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > >
> > > The connection is set up via QMI. The messages are either sent via
> > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > >
> > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > >
> > > Simply put, supporting all these in userspace like ModemManager
> > > is a mess (Aleksander can probably confirm).
> > > It would be nice if this could be simplified through the WWAN subsystem.
> > >
> > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > > ---
> > >
> > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > > I just took that over from someone else. Realistically speaking, the
> > > current approach isn't too different from the UCI "backdoor interface"
> > > approach that was rejected for MHI...
> > >
> > > I kind of expected that I can just trivially copy some code from
> > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > > device. But it looks like the abstractions in wwan_core are kind of
> > > getting in the way here... As far as I can tell, they don't really fit
> > > together with the RPMSG interface.
> > >
> > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > > get notified when the TX queue is full or no longer full so I can call
> > > wwan_port_txon/off().
> > >
> > > Any suggestions or other thoughts?
> >
> > It would be indeed nice to get this in the WWAN framework.
> > I don't know much about rpmsg but I think it is straightforward for
> > the RX path, the ept_cb can simply forward the buffers to
> > wwan_port_rx.
>
> Right, that part should be straightforward.
>
> > For tx, simply call rpmsg_trysend() in the wwan tx
> > callback and don't use the txon/off helpers. In short, keep it simple
> > and check if you observe any issues.
> >
>
> I'm not sure that's a good idea. This sounds like exactly the kind of
> thing that might explode later just because I don't manage to get the
> TX queue full in my tests. In that case, writing to the WWAN char dev
> would not block, even if O_NONBLOCK is not set.

Right, if you think it could be a problem, you can always implement a
more complex solution like calling rpmsg_send from a
workqueue/kthread, and only re-enable tx once rpmsg_send returns.

>
> But I think you're right that it's probably easiest if I start with
> that, see if I can get anything working at all ...
>
> > And for sure you can propose changes in the WWAN framework if you
> > think something is missing to support your specific case.
> >
>
> ... and then we can discuss that further on a RFC PATCH or something
> like that. Does that sound good to you?

Yes, you can submit the series, no need to be RFC IMHO, this thread is
already your RFC.

Regards,
Loic

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Stephan Gerhold]

Hi Loic,

On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > > I've been thinking about creating some sort of "RPMSG" driver for the
> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > > >
> > > > It's easy to confuse all the different approaches that Qualcomm has to
> > > > talk to their modems, so I will first try to briefly give an overview
> > > > about those that I'm familiar with:
> > > >
> > > > ---
> > > > There is USB and MHI that are mainly used to talk to "external" modems.
> > > >
> > > > For the integrated modems in many Qualcomm SoCs there is typically
> > > > a separate control and data path. They are not really related to each
> > > > other (e.g. currently no common parent device in sysfs).
> > > >
> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > > >
> > > > The connection is set up via QMI. The messages are either sent via
> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > > >
> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > > >
> > > > Simply put, supporting all these in userspace like ModemManager
> > > > is a mess (Aleksander can probably confirm).
> > > > It would be nice if this could be simplified through the WWAN subsystem.
> > > >
> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > > > ---
> > > >
> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > > > I just took that over from someone else. Realistically speaking, the
> > > > current approach isn't too different from the UCI "backdoor interface"
> > > > approach that was rejected for MHI...
> > > >
> > > > I kind of expected that I can just trivially copy some code from
> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > > > device. But it looks like the abstractions in wwan_core are kind of
> > > > getting in the way here... As far as I can tell, they don't really fit
> > > > together with the RPMSG interface.
> > > >
> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > > > get notified when the TX queue is full or no longer full so I can call
> > > > wwan_port_txon/off().
> > > >
> > > > Any suggestions or other thoughts?
> > >
> > > It would be indeed nice to get this in the WWAN framework.
> > > I don't know much about rpmsg but I think it is straightforward for
> > > the RX path, the ept_cb can simply forward the buffers to
> > > wwan_port_rx.
> >
> > Right, that part should be straightforward.
> >
> > > For tx, simply call rpmsg_trysend() in the wwan tx
> > > callback and don't use the txon/off helpers. In short, keep it simple
> > > and check if you observe any issues.
> > >
> >
> > I'm not sure that's a good idea. This sounds like exactly the kind of
> > thing that might explode later just because I don't manage to get the
> > TX queue full in my tests. In that case, writing to the WWAN char dev
> > would not block, even if O_NONBLOCK is not set.
> 
> Right, if you think it could be a problem, you can always implement a
> more complex solution like calling rpmsg_send from a
> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> 

I did run into trouble when I tried to stream lots of data into the WWAN
char device (e.g. using dd). However, in practice (with ModemManager) 
I did not manage to cause such issues yet. Personally, I think it's
something we should get right, just to avoid trouble later
(like "modem suddenly stops working").

Right now I extended the WWAN port ops a bit so I tells me if the write
should be non-blocking or blocking and so I can call rpmsg_poll(...).

But having some sort of workqueue also sounds like it could work quite
well, thanks for the suggestion! Will think about it some more, or
I might post what I have right now so you can take a look.

> >
> > But I think you're right that it's probably easiest if I start with
> > that, see if I can get anything working at all ...
> >
> > > And for sure you can propose changes in the WWAN framework if you
> > > think something is missing to support your specific case.
> > >
> >
> > ... and then we can discuss that further on a RFC PATCH or something
> > like that. Does that sound good to you?
> 
> Yes, you can submit the series, no need to be RFC IMHO, this thread is
> already your RFC.
> 

I kind of see "RFC" like a "I'm not sure if the approach taken here is
really a good idea" and my current patch set currently still fits that
criteria. But at the end it's just a strange prefix for the mail subject
so it shouldn't matter too much. :)

Thanks!
Stephan

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Toke Høiland-Jørgensen]

Stephan Gerhold <stephan@gerhold.net> writes:

> Hi Loic,
>
> On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
>> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
>> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
>> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
>> > > > I've been thinking about creating some sort of "RPMSG" driver for the
>> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
>> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
>> > > >
>> > > > It's easy to confuse all the different approaches that Qualcomm has to
>> > > > talk to their modems, so I will first try to briefly give an overview
>> > > > about those that I'm familiar with:
>> > > >
>> > > > ---
>> > > > There is USB and MHI that are mainly used to talk to "external" modems.
>> > > >
>> > > > For the integrated modems in many Qualcomm SoCs there is typically
>> > > > a separate control and data path. They are not really related to each
>> > > > other (e.g. currently no common parent device in sysfs).
>> > > >
>> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
>> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
>> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
>> > > >
>> > > > The connection is set up via QMI. The messages are either sent via
>> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
>> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
>> > > >
>> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
>> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
>> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
>> > > >
>> > > > Simply put, supporting all these in userspace like ModemManager
>> > > > is a mess (Aleksander can probably confirm).
>> > > > It would be nice if this could be simplified through the WWAN subsystem.
>> > > >
>> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
>> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
>> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
>> > > > ---
>> > > >
>> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
>> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
>> > > > I just took that over from someone else. Realistically speaking, the
>> > > > current approach isn't too different from the UCI "backdoor interface"
>> > > > approach that was rejected for MHI...
>> > > >
>> > > > I kind of expected that I can just trivially copy some code from
>> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
>> > > > device. But it looks like the abstractions in wwan_core are kind of
>> > > > getting in the way here... As far as I can tell, they don't really fit
>> > > > together with the RPMSG interface.
>> > > >
>> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
>> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
>> > > > get notified when the TX queue is full or no longer full so I can call
>> > > > wwan_port_txon/off().
>> > > >
>> > > > Any suggestions or other thoughts?
>> > >
>> > > It would be indeed nice to get this in the WWAN framework.
>> > > I don't know much about rpmsg but I think it is straightforward for
>> > > the RX path, the ept_cb can simply forward the buffers to
>> > > wwan_port_rx.
>> >
>> > Right, that part should be straightforward.
>> >
>> > > For tx, simply call rpmsg_trysend() in the wwan tx
>> > > callback and don't use the txon/off helpers. In short, keep it simple
>> > > and check if you observe any issues.
>> > >
>> >
>> > I'm not sure that's a good idea. This sounds like exactly the kind of
>> > thing that might explode later just because I don't manage to get the
>> > TX queue full in my tests. In that case, writing to the WWAN char dev
>> > would not block, even if O_NONBLOCK is not set.
>> 
>> Right, if you think it could be a problem, you can always implement a
>> more complex solution like calling rpmsg_send from a
>> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
>> 
>
> I did run into trouble when I tried to stream lots of data into the WWAN
> char device (e.g. using dd). However, in practice (with ModemManager) 
> I did not manage to cause such issues yet. Personally, I think it's
> something we should get right, just to avoid trouble later
> (like "modem suddenly stops working").
>
> Right now I extended the WWAN port ops a bit so I tells me if the write
> should be non-blocking or blocking and so I can call rpmsg_poll(...).
>
> But having some sort of workqueue also sounds like it could work quite
> well, thanks for the suggestion! Will think about it some more, or
> I might post what I have right now so you can take a look.

How big are those hardware TXQs? Just pushing packets to the hardware
until it overflows sounds like a recipe for absolutely terrible
bufferbloat... That would be bad!

-Toke

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Stephan Gerhold]

On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
> Stephan Gerhold <stephan@gerhold.net> writes:
> > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
> >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> >> > > > I've been thinking about creating some sort of "RPMSG" driver for the
> >> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> >> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> >> > > >
> >> > > > It's easy to confuse all the different approaches that Qualcomm has to
> >> > > > talk to their modems, so I will first try to briefly give an overview
> >> > > > about those that I'm familiar with:
> >> > > >
> >> > > > ---
> >> > > > There is USB and MHI that are mainly used to talk to "external" modems.
> >> > > >
> >> > > > For the integrated modems in many Qualcomm SoCs there is typically
> >> > > > a separate control and data path. They are not really related to each
> >> > > > other (e.g. currently no common parent device in sysfs).
> >> > > >
> >> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> >> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> >> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> >> > > >
> >> > > > The connection is set up via QMI. The messages are either sent via
> >> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> >> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> >> > > >
> >> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> >> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> >> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> >> > > >
> >> > > > Simply put, supporting all these in userspace like ModemManager
> >> > > > is a mess (Aleksander can probably confirm).
> >> > > > It would be nice if this could be simplified through the WWAN subsystem.
> >> > > >
> >> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> >> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
> >> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> >> > > > ---
> >> > > >
> >> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> >> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> >> > > > I just took that over from someone else. Realistically speaking, the
> >> > > > current approach isn't too different from the UCI "backdoor interface"
> >> > > > approach that was rejected for MHI...
> >> > > >
> >> > > > I kind of expected that I can just trivially copy some code from
> >> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> >> > > > device. But it looks like the abstractions in wwan_core are kind of
> >> > > > getting in the way here... As far as I can tell, they don't really fit
> >> > > > together with the RPMSG interface.
> >> > > >
> >> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> >> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> >> > > > get notified when the TX queue is full or no longer full so I can call
> >> > > > wwan_port_txon/off().
> >> > > >
> >> > > > Any suggestions or other thoughts?
> >> > >
> >> > > It would be indeed nice to get this in the WWAN framework.
> >> > > I don't know much about rpmsg but I think it is straightforward for
> >> > > the RX path, the ept_cb can simply forward the buffers to
> >> > > wwan_port_rx.
> >> >
> >> > Right, that part should be straightforward.
> >> >
> >> > > For tx, simply call rpmsg_trysend() in the wwan tx
> >> > > callback and don't use the txon/off helpers. In short, keep it simple
> >> > > and check if you observe any issues.
> >> > >
> >> >
> >> > I'm not sure that's a good idea. This sounds like exactly the kind of
> >> > thing that might explode later just because I don't manage to get the
> >> > TX queue full in my tests. In that case, writing to the WWAN char dev
> >> > would not block, even if O_NONBLOCK is not set.
> >> 
> >> Right, if you think it could be a problem, you can always implement a
> >> more complex solution like calling rpmsg_send from a
> >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> >> 
> >
> > I did run into trouble when I tried to stream lots of data into the WWAN
> > char device (e.g. using dd). However, in practice (with ModemManager) 
> > I did not manage to cause such issues yet. Personally, I think it's
> > something we should get right, just to avoid trouble later
> > (like "modem suddenly stops working").
> >
> > Right now I extended the WWAN port ops a bit so I tells me if the write
> > should be non-blocking or blocking and so I can call rpmsg_poll(...).
> >
> > But having some sort of workqueue also sounds like it could work quite
> > well, thanks for the suggestion! Will think about it some more, or
> > I might post what I have right now so you can take a look.
> 
> How big are those hardware TXQs? Just pushing packets to the hardware
> until it overflows sounds like a recipe for absolutely terrible
> bufferbloat... That would be bad!
> 

For reference, we're not really talking about "hardware" TXQs here.
As far as I understand, the RPMSG channels on Qualcomm devices are
mostly just a firmware convention for communicating between different
CPUs/DSPs via shared memory.

The packets are copied into some kind of shared FIFO/ring buffer
per channel, with varying sizes. On my test device, the firmware
allcates 1024 bytes for the QMI channel and 8192 bytes
for the AT channel.

I'm not sure how this would cause any kind of overflow/bufferbloat.
The remote side (e.g. modem DSP) is notified separately for every packet
that is sent. If we're really writing more quickly than the remote side
will read, rpmsg_send() will block and therefore the client will
block as well (since tx was disabled before calling rpmsg_send()).

Thanks,
Stephan

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Loic Poulain]

Hi Setphan,

On Mon, 7 Jun 2021 at 13:44, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
> > Stephan Gerhold <stephan@gerhold.net> writes:
> > > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> > >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
> > >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > >> > > > I've been thinking about creating some sort of "RPMSG" driver for the
> > >> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > >> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > >> > > >
> > >> > > > It's easy to confuse all the different approaches that Qualcomm has to
> > >> > > > talk to their modems, so I will first try to briefly give an overview
> > >> > > > about those that I'm familiar with:
> > >> > > >
> > >> > > > ---
> > >> > > > There is USB and MHI that are mainly used to talk to "external" modems.
> > >> > > >
> > >> > > > For the integrated modems in many Qualcomm SoCs there is typically
> > >> > > > a separate control and data path. They are not really related to each
> > >> > > > other (e.g. currently no common parent device in sysfs).
> > >> > > >
> > >> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > >> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > >> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > >> > > >
> > >> > > > The connection is set up via QMI. The messages are either sent via
> > >> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > >> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > >> > > >
> > >> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > >> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > >> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > >> > > >
> > >> > > > Simply put, supporting all these in userspace like ModemManager
> > >> > > > is a mess (Aleksander can probably confirm).
> > >> > > > It would be nice if this could be simplified through the WWAN subsystem.
> > >> > > >
> > >> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > >> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > >> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > >> > > > ---
> > >> > > >
> > >> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > >> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > >> > > > I just took that over from someone else. Realistically speaking, the
> > >> > > > current approach isn't too different from the UCI "backdoor interface"
> > >> > > > approach that was rejected for MHI...
> > >> > > >
> > >> > > > I kind of expected that I can just trivially copy some code from
> > >> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > >> > > > device. But it looks like the abstractions in wwan_core are kind of
> > >> > > > getting in the way here... As far as I can tell, they don't really fit
> > >> > > > together with the RPMSG interface.
> > >> > > >
> > >> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > >> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > >> > > > get notified when the TX queue is full or no longer full so I can call
> > >> > > > wwan_port_txon/off().
> > >> > > >
> > >> > > > Any suggestions or other thoughts?
> > >> > >
> > >> > > It would be indeed nice to get this in the WWAN framework.
> > >> > > I don't know much about rpmsg but I think it is straightforward for
> > >> > > the RX path, the ept_cb can simply forward the buffers to
> > >> > > wwan_port_rx.
> > >> >
> > >> > Right, that part should be straightforward.
> > >> >
> > >> > > For tx, simply call rpmsg_trysend() in the wwan tx
> > >> > > callback and don't use the txon/off helpers. In short, keep it simple
> > >> > > and check if you observe any issues.
> > >> > >
> > >> >
> > >> > I'm not sure that's a good idea. This sounds like exactly the kind of
> > >> > thing that might explode later just because I don't manage to get the
> > >> > TX queue full in my tests. In that case, writing to the WWAN char dev
> > >> > would not block, even if O_NONBLOCK is not set.
> > >>
> > >> Right, if you think it could be a problem, you can always implement a
> > >> more complex solution like calling rpmsg_send from a
> > >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> > >>
> > >
> > > I did run into trouble when I tried to stream lots of data into the WWAN
> > > char device (e.g. using dd). However, in practice (with ModemManager)
> > > I did not manage to cause such issues yet. Personally, I think it's
> > > something we should get right, just to avoid trouble later
> > > (like "modem suddenly stops working").
> > >
> > > Right now I extended the WWAN port ops a bit so I tells me if the write
> > > should be non-blocking or blocking and so I can call rpmsg_poll(...).

OK, but note that poll seems to be an optional rpmsg ops, rpmsg_poll
can be a no-op and so would not guarantee that EPOLL_OUT will ever be
set.


Loic

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Stephan Gerhold]

Hi,

On Mon, Jun 07, 2021 at 02:16:10PM +0200, Loic Poulain wrote:
> On Mon, 7 Jun 2021 at 13:44, Stephan Gerhold <stephan@gerhold.net> wrote:
> >
> > On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
> > > Stephan Gerhold <stephan@gerhold.net> writes:
> > > > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> > > >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > > >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > >> > > > I've been thinking about creating some sort of "RPMSG" driver for the
> > > >> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > > >> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > > >> > > >
> > > >> > > > It's easy to confuse all the different approaches that Qualcomm has to
> > > >> > > > talk to their modems, so I will first try to briefly give an overview
> > > >> > > > about those that I'm familiar with:
> > > >> > > >
> > > >> > > > ---
> > > >> > > > There is USB and MHI that are mainly used to talk to "external" modems.
> > > >> > > >
> > > >> > > > For the integrated modems in many Qualcomm SoCs there is typically
> > > >> > > > a separate control and data path. They are not really related to each
> > > >> > > > other (e.g. currently no common parent device in sysfs).
> > > >> > > >
> > > >> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > > >> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > > >> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > > >> > > >
> > > >> > > > The connection is set up via QMI. The messages are either sent via
> > > >> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > > >> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > > >> > > >
> > > >> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > > >> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > > >> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > > >> > > >
> > > >> > > > Simply put, supporting all these in userspace like ModemManager
> > > >> > > > is a mess (Aleksander can probably confirm).
> > > >> > > > It would be nice if this could be simplified through the WWAN subsystem.
> > > >> > > >
> > > >> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > > >> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > > >> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > > >> > > > ---
> > > >> > > >
> > > >> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > > >> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > > >> > > > I just took that over from someone else. Realistically speaking, the
> > > >> > > > current approach isn't too different from the UCI "backdoor interface"
> > > >> > > > approach that was rejected for MHI...
> > > >> > > >
> > > >> > > > I kind of expected that I can just trivially copy some code from
> > > >> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > > >> > > > device. But it looks like the abstractions in wwan_core are kind of
> > > >> > > > getting in the way here... As far as I can tell, they don't really fit
> > > >> > > > together with the RPMSG interface.
> > > >> > > >
> > > >> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > > >> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > > >> > > > get notified when the TX queue is full or no longer full so I can call
> > > >> > > > wwan_port_txon/off().
> > > >> > > >
> > > >> > > > Any suggestions or other thoughts?
> > > >> > >
> > > >> > > It would be indeed nice to get this in the WWAN framework.
> > > >> > > I don't know much about rpmsg but I think it is straightforward for
> > > >> > > the RX path, the ept_cb can simply forward the buffers to
> > > >> > > wwan_port_rx.
> > > >> >
> > > >> > Right, that part should be straightforward.
> > > >> >
> > > >> > > For tx, simply call rpmsg_trysend() in the wwan tx
> > > >> > > callback and don't use the txon/off helpers. In short, keep it simple
> > > >> > > and check if you observe any issues.
> > > >> > >
> > > >> >
> > > >> > I'm not sure that's a good idea. This sounds like exactly the kind of
> > > >> > thing that might explode later just because I don't manage to get the
> > > >> > TX queue full in my tests. In that case, writing to the WWAN char dev
> > > >> > would not block, even if O_NONBLOCK is not set.
> > > >>
> > > >> Right, if you think it could be a problem, you can always implement a
> > > >> more complex solution like calling rpmsg_send from a
> > > >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> > > >>
> > > >
> > > > I did run into trouble when I tried to stream lots of data into the WWAN
> > > > char device (e.g. using dd). However, in practice (with ModemManager)
> > > > I did not manage to cause such issues yet. Personally, I think it's
> > > > something we should get right, just to avoid trouble later
> > > > (like "modem suddenly stops working").
> > > >
> > > > Right now I extended the WWAN port ops a bit so I tells me if the write
> > > > should be non-blocking or blocking and so I can call rpmsg_poll(...).
> 
> OK, but note that poll seems to be an optional rpmsg ops, rpmsg_poll
> can be a no-op and so would not guarantee that EPOLL_OUT will ever be
> set.
> 

Wouldn't that be more a limitation of the driver that provides the rpmsg
ops then? If rpmsg_send(...) may block, it should also be possible to
implement rpmsg_poll(...). Right now my implementation basically behaves
mostly like the existing rpmsg-char. That's what has been used on
several devices for more than a year and it works quite well.

Granted, so far testing was mostly done with the "qcom_smd" RPMSG driver
(which is likely going to be the primary use case for my driver for now).
This one also happens to be the only one that implements rpmsg_poll()
at the moment...

I asked some other people to test it for AT messages with "glink" on
newer Qualcomm SoCs and that doesn't work that well yet. However, as far
as I can tell most of the trouble seems to be caused by various strange
bugs in the "glink" driver.
(See drivers/rpmsg for the drivers I'm referring to...)

---

One of the main potential problems I see with some kind of workqueue
approach is that all writes would seemingly succeed for the client.
When the extra thread is done with rpmsg_send(...) we will have already
returned from the char dev write(...) syscall, without a way to report
the error. Perhaps that's unlikely though and we don't need to worry
about that too much?

I'm not really sure which one of the solution is best at the end.
I think it doesn't make too much of a difference at the end, both can
work well and both have some advantages/disadvantages in certain fairly
unlikely situations.

Stephan

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Toke Høiland-Jørgensen]

Stephan Gerhold <stephan@gerhold.net> writes:

> On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
>> Stephan Gerhold <stephan@gerhold.net> writes:
>> > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
>> >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
>> >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
>> >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
>> >> > > > I've been thinking about creating some sort of "RPMSG" driver for the
>> >> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
>> >> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
>> >> > > >
>> >> > > > It's easy to confuse all the different approaches that Qualcomm has to
>> >> > > > talk to their modems, so I will first try to briefly give an overview
>> >> > > > about those that I'm familiar with:
>> >> > > >
>> >> > > > ---
>> >> > > > There is USB and MHI that are mainly used to talk to "external" modems.
>> >> > > >
>> >> > > > For the integrated modems in many Qualcomm SoCs there is typically
>> >> > > > a separate control and data path. They are not really related to each
>> >> > > > other (e.g. currently no common parent device in sysfs).
>> >> > > >
>> >> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
>> >> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
>> >> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
>> >> > > >
>> >> > > > The connection is set up via QMI. The messages are either sent via
>> >> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
>> >> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
>> >> > > >
>> >> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
>> >> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
>> >> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
>> >> > > >
>> >> > > > Simply put, supporting all these in userspace like ModemManager
>> >> > > > is a mess (Aleksander can probably confirm).
>> >> > > > It would be nice if this could be simplified through the WWAN subsystem.
>> >> > > >
>> >> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
>> >> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
>> >> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
>> >> > > > ---
>> >> > > >
>> >> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
>> >> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
>> >> > > > I just took that over from someone else. Realistically speaking, the
>> >> > > > current approach isn't too different from the UCI "backdoor interface"
>> >> > > > approach that was rejected for MHI...
>> >> > > >
>> >> > > > I kind of expected that I can just trivially copy some code from
>> >> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
>> >> > > > device. But it looks like the abstractions in wwan_core are kind of
>> >> > > > getting in the way here... As far as I can tell, they don't really fit
>> >> > > > together with the RPMSG interface.
>> >> > > >
>> >> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
>> >> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
>> >> > > > get notified when the TX queue is full or no longer full so I can call
>> >> > > > wwan_port_txon/off().
>> >> > > >
>> >> > > > Any suggestions or other thoughts?
>> >> > >
>> >> > > It would be indeed nice to get this in the WWAN framework.
>> >> > > I don't know much about rpmsg but I think it is straightforward for
>> >> > > the RX path, the ept_cb can simply forward the buffers to
>> >> > > wwan_port_rx.
>> >> >
>> >> > Right, that part should be straightforward.
>> >> >
>> >> > > For tx, simply call rpmsg_trysend() in the wwan tx
>> >> > > callback and don't use the txon/off helpers. In short, keep it simple
>> >> > > and check if you observe any issues.
>> >> > >
>> >> >
>> >> > I'm not sure that's a good idea. This sounds like exactly the kind of
>> >> > thing that might explode later just because I don't manage to get the
>> >> > TX queue full in my tests. In that case, writing to the WWAN char dev
>> >> > would not block, even if O_NONBLOCK is not set.
>> >> 
>> >> Right, if you think it could be a problem, you can always implement a
>> >> more complex solution like calling rpmsg_send from a
>> >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
>> >> 
>> >
>> > I did run into trouble when I tried to stream lots of data into the WWAN
>> > char device (e.g. using dd). However, in practice (with ModemManager) 
>> > I did not manage to cause such issues yet. Personally, I think it's
>> > something we should get right, just to avoid trouble later
>> > (like "modem suddenly stops working").
>> >
>> > Right now I extended the WWAN port ops a bit so I tells me if the write
>> > should be non-blocking or blocking and so I can call rpmsg_poll(...).
>> >
>> > But having some sort of workqueue also sounds like it could work quite
>> > well, thanks for the suggestion! Will think about it some more, or
>> > I might post what I have right now so you can take a look.
>> 
>> How big are those hardware TXQs? Just pushing packets to the hardware
>> until it overflows sounds like a recipe for absolutely terrible
>> bufferbloat... That would be bad!
>> 
>
> For reference, we're not really talking about "hardware" TXQs here.
> As far as I understand, the RPMSG channels on Qualcomm devices are
> mostly just a firmware convention for communicating between different
> CPUs/DSPs via shared memory.
>
> The packets are copied into some kind of shared FIFO/ring buffer
> per channel, with varying sizes. On my test device, the firmware
> allcates 1024 bytes for the QMI channel and 8192 bytes
> for the AT channel.
>
> I'm not sure how this would cause any kind of overflow/bufferbloat.
> The remote side (e.g. modem DSP) is notified separately for every packet
> that is sent. If we're really writing more quickly than the remote side
> will read, rpmsg_send() will block and therefore the client will
> block as well (since tx was disabled before calling rpmsg_send()).

Hmm, okay, if this is just control channel traffic and the buffers are
no bigger than that maybe this is not such a huge issue. As long as the
client (which I guess is whichever application is trying to control the
modem?) can block and back off, so it won't just keep queuing up
commands faster than the modem can process them...

-Toke

Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem

[Loic Poulain]

HI Stephan,

On Mon, 7 Jun 2021 at 14:49, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> Hi,
>
> On Mon, Jun 07, 2021 at 02:16:10PM +0200, Loic Poulain wrote:
> > On Mon, 7 Jun 2021 at 13:44, Stephan Gerhold <stephan@gerhold.net> wrote:
> > >
> > > On Mon, Jun 07, 2021 at 01:23:18PM +0200, Toke Høiland-Jørgensen wrote:
> > > > Stephan Gerhold <stephan@gerhold.net> writes:
> > > > > On Mon, Jun 07, 2021 at 11:27:07AM +0200, Loic Poulain wrote:
> > > > >> On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > > >> > On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > > > >> > > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > > >> > > > I've been thinking about creating some sort of "RPMSG" driver for the
> > > > >> > > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > > > >> > > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > > > >> > > >
> > > > >> > > > It's easy to confuse all the different approaches that Qualcomm has to
> > > > >> > > > talk to their modems, so I will first try to briefly give an overview
> > > > >> > > > about those that I'm familiar with:
> > > > >> > > >
> > > > >> > > > ---
> > > > >> > > > There is USB and MHI that are mainly used to talk to "external" modems.
> > > > >> > > >
> > > > >> > > > For the integrated modems in many Qualcomm SoCs there is typically
> > > > >> > > > a separate control and data path. They are not really related to each
> > > > >> > > > other (e.g. currently no common parent device in sysfs).
> > > > >> > > >
> > > > >> > > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > > > >> > > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > > > >> > > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > > > >> > > >
> > > > >> > > > The connection is set up via QMI. The messages are either sent via
> > > > >> > > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > > > >> > > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > > > >> > > >
> > > > >> > > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > > > >> > > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > > > >> > > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > > > >> > > >
> > > > >> > > > Simply put, supporting all these in userspace like ModemManager
> > > > >> > > > is a mess (Aleksander can probably confirm).
> > > > >> > > > It would be nice if this could be simplified through the WWAN subsystem.
> > > > >> > > >
> > > > >> > > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > > > >> > > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > > > >> > > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > > > >> > > > ---
> > > > >> > > >
> > > > >> > > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > > > >> > > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > > > >> > > > I just took that over from someone else. Realistically speaking, the
> > > > >> > > > current approach isn't too different from the UCI "backdoor interface"
> > > > >> > > > approach that was rejected for MHI...
> > > > >> > > >
> > > > >> > > > I kind of expected that I can just trivially copy some code from
> > > > >> > > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > > > >> > > > device. But it looks like the abstractions in wwan_core are kind of
> > > > >> > > > getting in the way here... As far as I can tell, they don't really fit
> > > > >> > > > together with the RPMSG interface.
> > > > >> > > >
> > > > >> > > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > > > >> > > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > > > >> > > > get notified when the TX queue is full or no longer full so I can call
> > > > >> > > > wwan_port_txon/off().
> > > > >> > > >
> > > > >> > > > Any suggestions or other thoughts?
> > > > >> > >
> > > > >> > > It would be indeed nice to get this in the WWAN framework.
> > > > >> > > I don't know much about rpmsg but I think it is straightforward for
> > > > >> > > the RX path, the ept_cb can simply forward the buffers to
> > > > >> > > wwan_port_rx.
> > > > >> >
> > > > >> > Right, that part should be straightforward.
> > > > >> >
> > > > >> > > For tx, simply call rpmsg_trysend() in the wwan tx
> > > > >> > > callback and don't use the txon/off helpers. In short, keep it simple
> > > > >> > > and check if you observe any issues.
> > > > >> > >
> > > > >> >
> > > > >> > I'm not sure that's a good idea. This sounds like exactly the kind of
> > > > >> > thing that might explode later just because I don't manage to get the
> > > > >> > TX queue full in my tests. In that case, writing to the WWAN char dev
> > > > >> > would not block, even if O_NONBLOCK is not set.
> > > > >>
> > > > >> Right, if you think it could be a problem, you can always implement a
> > > > >> more complex solution like calling rpmsg_send from a
> > > > >> workqueue/kthread, and only re-enable tx once rpmsg_send returns.
> > > > >>
> > > > >
> > > > > I did run into trouble when I tried to stream lots of data into the WWAN
> > > > > char device (e.g. using dd). However, in practice (with ModemManager)
> > > > > I did not manage to cause such issues yet. Personally, I think it's
> > > > > something we should get right, just to avoid trouble later
> > > > > (like "modem suddenly stops working").
> > > > >
> > > > > Right now I extended the WWAN port ops a bit so I tells me if the write
> > > > > should be non-blocking or blocking and so I can call rpmsg_poll(...).
> >
> > OK, but note that poll seems to be an optional rpmsg ops, rpmsg_poll
> > can be a no-op and so would not guarantee that EPOLL_OUT will ever be
> > set.
> >
>
> Wouldn't that be more a limitation of the driver that provides the rpmsg
> ops then? If rpmsg_send(...) may block, it should also be possible to
> implement rpmsg_poll(...). Right now my implementation basically behaves
> mostly like the existing rpmsg-char. That's what has been used on
> several devices for more than a year and it works quite well.
>
> Granted, so far testing was mostly done with the "qcom_smd" RPMSG driver
> (which is likely going to be the primary use case for my driver for now).
> This one also happens to be the only one that implements rpmsg_poll()
> at the moment...
>
> I asked some other people to test it for AT messages with "glink" on
> newer Qualcomm SoCs and that doesn't work that well yet. However, as far
> as I can tell most of the trouble seems to be caused by various strange
> bugs in the "glink" driver.
> (See drivers/rpmsg for the drivers I'm referring to...)
>
> ---
>
> One of the main potential problems I see with some kind of workqueue
> approach is that all writes would seemingly succeed for the client.
> When the extra thread is done with rpmsg_send(...) we will have already
> returned from the char dev write(...) syscall, without a way to report
> the error. Perhaps that's unlikely though and we don't need to worry
> about that too much?
>
> I'm not really sure which one of the solution is best at the end.
> I think it doesn't make too much of a difference at the end, both can
> work well and both have some advantages/disadvantages in certain fairly
> unlikely situations.

Yes, feel free to submit the series for review.

Regards,
Loic